The field of the invention is the formation of vertical transistors in integrated circuit processing.
The vertical MOSFET device is advantageous for DRAM scaling (see Li et al, Int. Symp. VLSI Tech. Sys. App. 1999, p251; Radens et al, IEDM 2000, p.349; Gruening et al, IEDM 1999, p.25), since it maintains a desired array MOSFET channel length which is independent of the minimum lithographic feature size elsewhere on the chip. This results in properly scaled array MOSFETs for minimum lithographic feature sizes below 140 nm, which would be unattainable with DRAM arrays employing planar array MOSFETs. The longer channel length of the vertical MOSFET is decoupled from the minimum lithographic feature size, thus not impacting overall density.
Referring now to FIG. 1, there is shown a portion of an integrated circuit containing a DRAM cell 80 formed in silicon substrate 10 and including a vertical transistor 100 and a deep trench capacitor 30. The arrows at the top of the Figure indicate the prior art method of making a laterally uniform implant to adjust the threshold voltage. The implant extends across the entire silicon region that forms the body of the device, so that the dopant concentration is uniform through out the body. A problem known to the art is that a change in the body substrate or p-well bias in the vertical transistor 100 produces a change in the threshold voltage Vt.
In one example, a DRAM having a nominal 110 nm groundrule had a change in Vt of 460 mV in response to a change in P-Well bias from xe2x88x920.5V to xe2x88x922.0V and a change of 160 mV for a change in P-well bias from xe2x88x920.5V to xe2x88x921.0V. This is an extremely high body effect that causes significant performance degradation for DRAM circuit operating conditions such as write-back, which require charge transfer between bitline and storage capacitor. It would be highly advantageous to find a way to design the vertical device such that the body effect is minimized, but those skilled in the art did not understand the reasons for this high bias sensitivity and consequently were not able to devise an economical solution to this problem.